AB0708 ANALYSIS OF THE PERFORMANCE OF COMMERCIAL TESTS IN THE DETERMINATION OF MYOSITIS-SPECIFIC AND MYOSITIS-ASSOCIATED AUTOANTIBODIES: PREVALENCE AND CORRELATION WITH DIAGNOSIS OF IIM
Authors: E. Pedretti, F. Angeli, M. Fredi, I. Cavazzana, L. Andreoli, E. Garrafa, F. Franceschini
Journal: Annals of the Rheumatic Diseases (2022)

Background Idiopathic Inflammatory Myopathies (IIM) are a group of disorders sharing the common feature of immune-mediated muscle injury. In IIM autoantibodies specific for (MSA) or associated to (MAA) myositis are present in 60-65% of cases (1). Although only anti-Jo-1 (2) are included in the EULAR 2017 criteria, the presence of MSA allows to identify clinical and prognostic phenotypes. Objectives Aim of the work is to evaluate the positivity rate on sera analyzed by a line blot (LB) immunoassay able to detect MSA or MAA, the associated clinical diagnoses and the consistency with the antibody specificity. Methods 267 sera were analyzed using a commercial LB (Autoimmune Inflammatory Myopathies 16 Ag Profile EUROLINE) between March 2017 and March 2021. The diagnosis was identified by analyzing the medical records. Results A positivity for at least one MSA/MAA was found in 118/267 sera (44.2%); MAA isolated in 22% of cases and a double/multiple positivity for MSA in 12.7%. The distribution of antibody positivity and the associated clinical diagnoses are shown in Table 1. Table 1. Autoantibody distribution and associated clinical diagnosis; for some patients there are multiple positivity Autoantibody specificity Consistent diagnosis Total positivity n. (% ) Isolated positivity n. (% ) Concomitant presence of at least one MAA n. (% ) Concomitant presence of MSA +/- MAA n. (% ) n. (% ) Type of IIM MSA Mi-2 27 (22.9) 16 (59.3) 2 (7.4) 9 (33.3) 5 (18.6) 2 DM/2 PM/1 ASS Tif-1γ 12 (10.2) 4 (33.3) 4 (33.3) 4 (33.3) 9 (75) 5 CAM/4 DM PL-7 11 (9.4) 4 (36.4) 3 (27.3) 4 (36.4) 5 (45.5) ASS NXP-2 8 (6.8) 3 (37.5) 1 (12.5) 4 (50) 3 (37.5) DM SRP 8 (6.8) 3 (37.5) 1 (12.5) 4 (50) 1 (12.5) PM Jo-1 8 (6.8) 2 (25) 3 (37.5) 3 (37.5) 6 (75) ASS PL-12 7 (5.9) 3 (42.9) 0 4 (57.1) 4 (57.1) ASS MDA5 5 (4.2) 3 (60) 0 2 (40) 4 (80) 2 CADM/1 DM/1 ASS Oj 4 (3.4) 3 (75) 0 1 (25) 3 (75) ILD SAE 3 (2.5) 2 (66.7) 1 (33.3) 0 2 (66.7) DM Ej 2 (1.7) 1 (50) 1 (50) 0 1 (50) ASS MAA Ro-52 33 (28) 12 (36.3) 2 (6.1) 19 (57.6) - PM-Scl 19 (16.1) 11 (57.9) 3 (15.8) 5 (26.3) - Ku 6 (5.1) 3 (50) 1 (16.7) 2 (33.3) - DM: Dermatoyositis, PM: Polymiositis, ASS: Antisynthetase Syndrome, CAM: Cancer-Associated Myositis, CADM: Clinical Amyopatic Dermatomyositis, ILD: Interstitial Lung Disease In our cohort, the most common antibodies were anti-Ro52kD, positive in 33 sera (28%), while the most frequent MSA were anti-Mi-2, positive in 27 sera (22.9%). MSA most frequently found as an isolated positivity were anti-Oj (75%), while anti-Jo-1 as an isolated positivity were present only in 25% of cases. Considering the 118 sera with at least one MSA/MAA a diagnosis of IIM was made in 52 cases (44%); in 11 cases others systemic autoimmune diseases were identified. The consistency between antibody specificity with the clinical diagnoses was analyzed: for some antibodies, as anti-Jo-1 (80%), anti-MDA5 (75%) and anti-Tif-1gamma (75%), LB results were consistent with clinical diagnoses; while for others, as anti-Mi-2 (18,6%) and anti-SRP (12,5%), the agreement was lower. Conclusion The use of LB in the diagnostic workout of IIM allowed the detection of MSA previously not found with routine methods. In our court low prevalence of anti-Jo-1 was found. This may be justified by the fact that anti-Jo-1 are included in the assays routinely used to identify anti-ENA antibodies, therefore these sera were not tested with an additional LB. With LB, a higher prevalence of double/multiple MSA positivity was found, compared to the 0.1% described with immunoprecipitation (IP) (3). LB give a semiquantitative result: low, medium or high titer, so the analysis of different titers, according with clinical data or IP, could be helpful in the interpretation of the co-existence of multiple MSA (4). References [1]Betteridge Z, et al. J Autoimmun.2019;101:48-55 [2]Lundberg IE, et al. Ann Rheum Dis.2017;76:1955-1964 [3]Cavazzana I, et al. J Immunol Methods.2016;433:1-5 [4]Cavazzana I, et al. Ann Rheum Dis.2020;79:e152 Disclosure of Interests None declared